Nitrogen is an important plant nutrient. In addition to phosphorous, potassium, and other nutrients, nitrogen is needed to support the growth and development of plant life. Some plants, such as legumes, through a symbiotic relationship with Rhizobium bacteria, fix elemental nitrogen from the atmosphere and fix this nitrogen into the soil. However, most plants grown to produce human and animal food require the use of nitrogen fertilizer in order to sustain their agricultural production.
The most widely used and agriculturally important high-analysis nitrogen fertilizer is urea, CO(NH2)2. While most of the urea currently produced is used as a fertilizer in its granular form, urea-based fluid fertilizers are also well known. As used herein, the term “fluid fertilizers” encompasses liquid fertilizers, i.e. aqueous solutions of fertilizers, and suspension fertilizers, i.e. fertilizer compositions which in addition to water and water-soluble components also contain insoluble components kept in suspension by a suspending agent, such as clay. Suspension fertilizers are excellent carriers for pesticides and micronutrients.
The most commonly known urea-based liquid fertilizer is an aqueous solution of urea and ammonium nitrate, referred to in the fertilizer trade as UAN solution. These fluid fertilizers are used on a variety of crops, such as corn and wheat. When applied to moist soil, the urea component of the fluid fertilizer becomes a source of ammonia as a result of hydrolysis catalyzed by urease, an enzyme produced by numerous fungi and bacteria. The process is fully disclosed in U.S. Pat. No. 5,364,438, which is hereby incorporated by reference. Unfortunately, the urease-catalyzed hydrolysis often converts the urea to ammonia more quickly than it can be absorbed by the soil, resulting in undesirable ammonia loss to the atmosphere through a process called volatilization.
Urease inhibitors slow down the conversion of urea to ammonia, extending the period of nitrogen release. One urease inhibitor is N-(n-butyl)thiophosphoric triamide (NBPT). When incorporated into a urea-based fertilizer, NBPT reduces the rate at which urea is hydrolyzed to ammonia. This allows the nutrient nitrogen to be available to the soil and plants over a longer period of time. NBPT is widely recognized as an effective urease inhibitor, but NBPT is notoriously difficult to handle, because industrial grade NBPT is a waxy, sticky, heat-sensitive and water-sensitive material. As a result, a solvent system was disclosed in U.S. Pat. No. 5,364,438, that allowed the solution of NBPT to be applied to UAN. However, this solution had stability problems as well as problems in delivering and metering into fluid fertilizer.
U.S. Pat. No. 5,352,265, which is hereby incorporated by reference, discloses a granular fertilizer comprising about 90 to 99% urea, 0.02 to 0.5% NBPT and about 0 to 2.2% DCD. The NBPT is added to molten urea as a concentrated solution in an amide solvent. The DCD is added to the urea melt as a solid. This granular product is made to apply directly to the field crop.
The present invention of a dry flowable additive, is prepared by coating a solid urea-formaldehyde polymer (UFP) with a solution or suspension of NBPT in a liquid solvent, preferably an amide solvent. Optionally, the coated UFP may be blended with solid DCD. Prior to application of the fertilizer to the field crop, the dry flowable additive is blended with a UAN solution or aqueous urea, to form the fluid urea-containing fertilizer composition, or blended with solid or molten urea to form a solid urea-based fertilizer. The present invention provides a fluid or solid fertilizer composition that is easy to handle and stable when stored.